Transformer, which is a device that transforms an electric current from one voltage to another voltage, is of great practical importance to the transport and distribution of electrical power. A transformer normally has a primary coil and a secondary coil wound around a core that is usually made of iron to increase corresponding magnetic field and flux. A heat transfer composition such as transformer oil is used to keep the primary and secondary coils and the core cool and transfer heat to the surrounding through radiator.
Heat transfer compositions and methods for transferring heat between a heat source and a heat sink may find many other applications in both heating and cooling, including transformers, refrigerators, air conditioners, computer processors, thermal storage systems, heating pipes, fuel cells, and hot water and steam systems. Heat transfer media include a wide range of liquid or phase change materials, including oils, water, aqueous brines, alcohols, glycols, ammonia, hydrocarbons, ethers, and various halogen derivatives of these materials, such as chlorofluorocarbons (CFCs), hydro chlorofluorocarbons (Huffs), and the like. These have been used alone or in combination with additives, such as refrigerant oil additives for lubrication and composites of fluids to affect boiling or freezing temperature. Such heat transfer compositions are used to transfer heat from one part of a system to another part of the system, or from one system to another system, typically from a heat source (e.g., an vehicle engine, boiler, computer chip, or refrigerator), to a heat sink, to effect cooling of the heat source, heating of the heat sink, or to remove unwanted heat generated by the heat source. The heat transfer medium provides a thermal path or channel between the heat source and the heat sink, and may be circulated through a loop system or other flow system to improve heat flow. For an operating transformer, the heat source includes at least the primary and secondary coils and the core, and the heat sink includes the radiator, which in turn, transfers heat to the surrounding environment.
A number of criteria have been proposed and used for selecting heat transfer media for specific applications. Exemplary criteria include the influence of temperature on heat transfer capacity and viscosity, high dielectric strength, chemical stability of a medium at a given range of temperature and the energy required to pump the medium through a heat transfer system, to name a few. Specific parameters describing the comparative performance of a heat transfer medium include density, thermal conductivity, specific heat, and kinematics viscosity. The maximization of the heat transfer capability of any heat transfer system is important to the overall energy efficiency, material resource minimization, and system costs. Transformer oil is no exception.
Currently transformer oil available in the market has served its purpose well, but it suffers from excessive maintenance and replacement costs, environmental jeopardy and catastrophic failure incidence directly traceable to overheating, which may be partially due to the fact that transformer oil itself is a poor thermal conductor; hence local hot spots may lead to cracking of the oil's molecular composition and insulation collapse (corona).
Other factors that affect the feasibility and performance of heat transfer media such as transformer oil include environmental impact, toxicity, flammability, physical state at normal operating temperature, and corrosive nature.
Thus, among other things, there is a need to develop new and improved heat transfer compositions and methods that are cost-effective and have better performance, which can be used as a heat transfer medium in transformers and in other applications where heat transfer is needed as discussed above.
One of the challenges in finding such new and improved heat transfer compositions and methods is how to enhance thermal conductivity of a heat transfer composition, which often is in a liquid phase and in a form of suspension, without compromising other needed properties and at a reasonable cost. Historically, most research on particle suspensions has considered particles of micron scale or larger. In the present context, prior work may have shown that particles with higher thermal conductivity than their surrounding liquid can increase the composite material's effective thermal conductivity. However, recent work on smaller particles with diameters in the order of 10 nm has shown that further enhancement, beyond that predicted by macroscopic theory, is possible. In the present invention, among other things, we developed and studied such suspensions in an effort to characterize important effects of particle type, size, and concentration in practical engineering fluids for transferring heat without compromising the dielectric strength or viscosity of the fluid(s).
A satisfying explanation for nonsocial size effects on thermal conductivity enhancement has not yet appeared. As nanofabrication technology improves, the availability of solid particles with smaller and smaller size has increased. One significant advantage of nanoparticle suspensions is their improved solubility due to small size. Results by Choi et al.1 on colloids with grains ranging from 10 nm to 40 nm exhibit good suspension stability for weeks and even months. Further, because heat transfer occurs at the surface interfaces between particles and liquids, larger surface-to-volume ratios are expected to improve thermal conductance. Experiments by Lee et al.2 demonstrate that smaller-sized (˜10 nm) Al2O3 particles increase the thermal conductivity of aqueous solutions by more than 20% as compared to 40 nm Al2O3 particles with an identical volume fraction. Given that the surface-to-volume ratio of 10 nm particles is 1000 greater than that of 10 μm particles, this difference likely begins to explain, at least qualitatively, the experimental observations. Recently, Eastman et al.3 conducted tests on 10 nm Cu particles in a glycol-based nano-particle suspension and found a 40% increase in thermal conductivity with only 0.3% particle volume fraction—a result that is much higher than that predicted by traditional theory. These results demonstrate the shortcomings of macroscopic models applied to nonsocial materials.
Nevertheless, new models are still needed to predict the thermal conductivity of nano-scale particle suspensions. And new and improved heat transfer compositions and methods that are cost-effective and have better performance need to be developed accordingly.